1. Field of the Invention
The present invention relates to an apparatus and process for the indirect heat drying and simultaneous pelletization of liquid sludge, such as sludge obtained from a waste water treatment process. The sludge is mechanically dewatered, thermally dried, and processed to obtain sterile pellets, uniform in particle size and moisture content. Thereafter, the pellets are suitable for use as a fertilizer, an additive to a fertilizer, or as a fuel.
2. Description of Related Art
Municipal waste water treatment creates sewage sludge in large amounts. Raw sewage received in waste water treatment facilities is treated by various known methods which generate the sludge. Thereafter, a problem remains in how to treat and dispose of the sludge in an environmentally safe, energy efficient and economical way. Sludge is presently disposed of in many ways, such as by direct land application of the sludge, composting the sludge, land filling the sludge, ocean filling the sludge, and drying and incinerating the sludge.
Municipal sludge is mostly a liquid containing 2-6% total solids. It typically contains inorganic and organic matter, nutrients such as nitrogen, phosphorous and potassium and traces of various metals. It may also contain pathogens, and, in some instances, constituents such as heavy metals and hazardous organics, depending upon the source of the raw sewage that has been treated.
In the known methods of treating municipal sludge, the sludge is treated to increase its solid content. The sludge can be dewatered by gravity, by mechanically dewatering the sludge, and by thermal treatment of the sludge. The water content of sludge includes intracellular water, capillar water, colloidal water and free water. Free water can generally be separated from the sludge by gravity. The capillar and colloidal water can be removed from the sludge, usually after chemical conditioning, by mechanical means such as centrifuges, belt presses, vacuum filters and the like. Intracellular water, on the other hand, generally needs to be removed by breaking the cell structure down by thermal treatment. When liquid sludge is dewatered or thickened by gravity, the sludge product obtained is 2-6% total solids. The solids content is increased by mechanical dewatering of the sludge to 15-30% total solids. When the liquid sludge is thermally dried, a product of 85-95% total solids is obtained. Just as important, the volume of the sludge decreases as the sludge is processed to increase its solids content.
The type of dewatering process that is selected for a particular waste treatment plant is based upon several considerations. The thermal treatment of sludge has the advantage of the greatest reduction in sludge volume (up to 98%), and also destroys or inactivates pathogenic organisms, rendering the sludge sterile. On the other hand, the thermal treatment of sludge requires special drying equipment and an energy source for generating the heat needed in drying the sludge.
When liquid sludge is merely dewatered and land applied or ocean dumped, then there is a risk of contaminating the land or ocean with pathogens that have not been destroyed or inactivated during the treatment process. Further, if the sludge is not sufficiently dewatered, then an increased material handling problem arises in the transportation of the sludge by truck, barge, train or the like. Therefore, the thermal treatment of municipal sludge has environmental advantages as well as material handling advantages over the disposal of municipal sludge that is merely dewatered or land applied in the liquid sludge state.
Two types of sludge thermal drying are known in the treatment of municipal sludge. In the first type of system, known as direct drying, hot gas is brought into direct contact and mixed with the sludge in a chamber to vaporize the water in the sludge directly. The problem with direct drying of liquid sludge is that a large volume of malodorous gas is generated in the drying process that subsequently needs to be treated in order to maintain an odorless treatment process.
Another known type of thermal treatment of municipal sludge uses an indirect sludge drying process wherein the sludge contacts a heated surface to vaporize the water contained therein. One known type of indirect dryer is illustrated schematically in FIG. 2. The sludge is dried by spreading it across metal surfaces heated by steam or thermal oil so that direct contact between the heating medium and the sludge is prevented. Thus, the heating medium remains clean and uncontaminated, and the water vapors and volatile matter that are extracted during the drying are of relatively low volume. The organic matter and nutrients remain in the dried sludge product that exits the dryer. Further, the sludge is exposed to a temperature of above 100.degree. C. for a sufficient time to inactivate or destroy any living organisms contained therein such as viruses, pathogens, etc. This type of indirect heat dryer is used in a process, for example, wherein the dried sludge product is fed to a fluidized bed furnace for incineration of the sludge product. Incineration of sludge on a continuous basis, however, has obvious environmental drawbacks, and is therefore undesirable in general.